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Dusted for Life's Fingerprints, Rocks Fail

Just when life began on Earth tells, in part, how easy or difficult it is for life to arise.

A 1996 report that researchers had found evidence of microbial life in 3.85-billion-year-old rocks near Greenland suggested that life was relatively easy.

At 3.85 billion years ago, the heavy bombardment of giant meteors from the early solar system was only beginning to tail off. Life could not survive the impacts, which repeatedly vaporized all of the oceans into steam. That indicated that life started on Earth as soon as conditions turned favorable.

That also suggested that life might have arisen several separate times, since the next giant meteor impact would have again boiled off the oceans and killed any organisms on the planet.

But new research reveals that the rocks, found on the island of Akilia off Greenland's west coast, probably do not contain evidence of life after all.

The evidence has always been indirect. Scientists have never seen any signs of fossils. The original claim was that the green and white layered rocks were of a type of sedimentary rock known as banded iron formations, indicating they had been formed at the bottom of an ocean, and that they contained high concentrations of light carbon, considered a fingerprint of life.

Carbon comes in two stable forms, or isotopes -- one has an extra neutron in the nucleus, making it heavier. Bacteria prefer eating molecules with the lighter carbon because they take less energy to digest. Thus, sediments containing bacterial remains tend to contain more light carbon.

But, scientists have had some doubts about the Akilia rocks because of their less-than-pristine state. After they formed, the rocks, riding the conveyor belt of continental drift, were pushed down into the earth and heated to 1,000 degrees Fahrenheit or more under very high pressures before re-emerging to the surface. Those conditions at least partly altered the rocks.

To Dr. Christopher M. Fedo, a professor of earth and environmental sciences at George Washington University in Washington, and Dr. Martin J. Whitehouse of the Swedish Museum of Natural History, the rocks did not look like banded iron formations at all. Unlike sedimentary rocks, some layers in the Akilia rocks are not of even thickness, bunching up in clumps in some places and thinning out in others.

The green layers, the researchers found, resemble basaltlike rock known as komatiite, which forms from molten lava, not ocean sediments. The white layers appear to be veins of quartz that later squirted into the komatiite, the scientists reported, and the high temperatures and pressures squeezed and twisted the minerals into the layered structure.

''Like taffy,'' Dr. Fedo said. ''You would end up with this repeated striped or banded rock.''

Dr. Fedo and Dr. Whitehouse reported their findings in the May 24 issue of the journal Science.

Dr. Gustaf Arrhenius, a professor of oceanography at Scripps Institution of Oceanography in San Diego and an author of the 1996 paper that claimed the signs of life in the Akilia rocks, agreed with the new analysis. But, he added, ''The question of carbon isotopic composition is something of a mystery.''

Some nonbiological chemical reactions are also known to produce concentrations of light carbon.

At present, discarding the Akilia rocks does not shift the prevailing timeline of life by much. The next oldest evidence for life is not much younger -- or very far away. On Greenland itself are 3.8-billion-year-old sedimentary rocks that Dr. Fedo says are true banded iron formations, with the relatively high amounts of the light carbon.

But if nonliving chemical reactions can also increase concentration of light carbon, some scientists think that this chemical fingerprint is insufficient proof and that the beginning of life could have been hundreds of millions of years later. ''I'm very skeptical of all the carbon isotope evidence,'' said Dr. James F. Kasting, a professor of geosciences at Penn State University.

If life began after the end of heavy meteor bombardment, about 3.8 billion years ago, then the advent of life may have a rare, unlikely confluence of chance events.